Telemetric measuring methods are, for instance, used for measuring a person's heart rate. Equipment solutions are usually such that the unit for measuring and transmitting the heart rate data is arranged around the person's chest as a transmitter belt, from which the measurement data is telemetrically transferred by means of inductive coupling to a receiver unit, often implemented as a receiver wristband on a person's wrist. In cycling, the receiver unit can be secured to a bicycle handlebar, for example.
The transmitters of heart rate measurement devices transmit a burst of approximately 5 kHz each time the transmitters detect an ECG signal. The transmitter circuit of the transmitter unit comprises a resonance unit of a coil and a capacitor, the resonance circuit being activated and controlled by the heart rate. The receiver unit computes the pulse frequency on the basis of the time difference between successive transmitted signals, i.e. the time difference of the bursts, the information to be transmitted, i.e. the heart rate, being thus included in the transmission coded in the interval between the pulses.
Currently, however, a need has arisen for telemetric transmission of measurement data from several different sensors, for instance heart rate, pedaling speed and pedaling cadence, to the same receiver. In the prior art, the sensors transmit a measurement signal as a pulse group. In a pulse group, three pulses form two intervals which differ from the intervals of the pulse groups of other variables. Such a 3-pulse-coding is called a 3-pulse transmission. The receiver unit identifies each measurement variable on the basis of the pulse group interval.
Many applications, however, require higher-rate data transfer than the 3-pulse transmission is able to offer. In addition, the receiver unit is only capable of distinguishing a limited number of sensors from each other due to the coding employed in the 3-pulse transmission.